This invention relates to a detecting system for sensing changes in permeability of a rotatable shaft to determine its speed and torque.
Systems have been developed having transducers for producing an alternating magnetic field adjacent to and within a rotating shaft and for measuring the changes in that magnetic field to provide information regarding different shaft characteristics, such as speed, torque and horsepower. It has been known that as a driven or loaded shaft is torsionally stressed, compression and tension occur in various portions of the shaft and this changes the shaft permeability and consequently the magnetic flux pattern in the shaft thereby to alter the magnetic field. A torque information signal may be developed from the altered or resulting magnetic field. Such a system is disclosed in U.S. Pat. No. 4,100,794, issued July 18, 1978 in the name of Edwin J. Meixner, and which is assigned to the assignee of the present invention. In addition, it is recognized in U.S. Pat. No. 4,100,794 that the magnetic field also varies as a function of the shaft speed due to the armature reaction in the shaft. Eddy currents will flow on the surface of the rotating shaft and produce a counter magnetic field which opposes and distorts the original magnetic field. The faster the rotational speed, the greater the eddy current flow and the greater the amount of field distortion. As a result, the modified magnetic field may also be used to generate a speed information signal.
The required alternating magnetic field is produced in the transducer in U.S. Pat. No. 4,100,794 by a circular magnetic core, having a series of radially extending poles, on which core is wound a single primary winding comprising series-connected coils, each of the coils being disposed on a respective one of the poles. An a-c voltage source is coupled to the primary winding for translating alternating current to the winding to develop an alternating magnetic flux adjacent to and within the shaft, which shaft extends through the center of the circular magnetic core. Since bidirectional current flow is needed in the primary winding, the driving circuitry to provide the necessary a-c excitation for that winding is relatively costly. In one arrangement, a transformer is employed to provide alternating current in the primary winding. In accordance with another scheme, a-c for the primary winding can be developed by converting d-c to a-c by means of a bridge-type inverter or a similar switching arrangement.
An improved excitation system for the transducer is disclosed in U.S. Pat. No. 4,106,334, issued Aug. 15, 1978 in the name of George H. Studtmann, and assigned to the present assignee. In the Studtmann patent, two multi-coil primary windings are wound in a bifilar arrangement on the magnetic core structure. Each of the radial poles therefore has two coils wound thereon, one coil from each of the two primary windings. The two coils on each pole are physically adjacent to each other, but are electrically isolated or insulated from each other. With this driving scheme, bidirectional current flow is not needed and a simple switching circuit may be utilized to supply, from a direct voltage source, direct current pulses alternately to the two primary windings. In this way, when a d-c pulse is translated to one primary winding the other primary winding will be ineffective and all of the poles will be magnetized with one polarity, whereas when a d-c pulse is then supplied to the other primary winding the first winding will be inoperative and all of the poles will be magnetized with the opposite polarity. As a consequence, the d-c pulses will produce an alternating magnetic flux or field in the central area of the magnetic core through which the rotating shaft extends.
While the approach in U.S. Pat. No. 4,106,334 constitutes an improvement over the arrangement in U.S. Pat. No. 4,100,794, the present invention is a meritorious advance over both of those patents, achieving substantially increased economy and simplification.